This invention relates to telemetry systems for use in cardiac pacemakers and other implantable medical devices, and more particularly to such telemetry systems employing pulse modulation.
Modern pacemakers conventionally employ some form of communications link for transmission and reception of information when implanted in the body of a patient. External programming devices are routinely used for remotely programming operating mode and other parameters affecting pacemaker operation, such as stimulation pulse rate, escape interval, refractory period, pulse width, pulse amplitude, and sensitivity. In addition, pacemakers commonly have the capability of transmitting status information as well as biological information via telemetry. Actual settings of operating mode and other programmed parameters may be ascertained in this manner, as may the condition of the pacemaker battery. Electrical activity within the heart, temperature, and other physiological parameters sensed by pacemakers have also been externally monitored via telemetry.
Examples of various telemetry methods and forms of modulation which have been employed for communicating non-invasively through the skin may be found in the following U.S. patents:
______________________________________ U.S. Pat. No. Inventor Issue Date Modulation* ______________________________________ 4,026,305 Brownlee et al. May 31, 1977 PIM 4,223,679 Schulman et al. Sep. 23, 1980 FM/FM, FM/AM 4,237,895 Johnson Dec. 9, 1980 PIM, PWM 4,281,664 Duggan Aug. 4, 1981 FM, FSK 4,409,984 Dick Oct. 18, 1983 FM 4,522,208 Buffet Jun. 11, 1985 PPM 4,543,953 Slocum et al. Oct. 1, 1985 PM, PSK 4,550,370 Baker Oct. 29, 1985 PIM 4,556,063 Thompson et al. Dec. 3, 1985 PIM 4,681,111 Silvian Jul. 21, 1987 FM, PSK, PWM, PPM, FSK 4,686,990 Moberg Aug. 18, 1987 PPM 4,741,340 Batina et al. May 3, 1988 PWM 4,757,816 Ryan et al. Jul. 9, 1988 PIM 4,791,936 Snell et al. Dec. 20, 1988 FM, PAM/PWM, PSK ______________________________________ *AM = Amplitude Modulation FM = Frequency Modulation FSK = Frequency Shift Keying PM = Phase Modulation PAM = Pulse Amplitude Modulation PIM = Pulse Interval Modulation PPM = Pulse Position Modulation PSK = Phase Shift Keying PWM = Pulse Width Modulation
Among those patents describing some form of pulse modulation, the patent to Moberg discloses a pacemaker battery test circuit which positions a marking pulse at a chronological position between successive stimulation pulses which depends on the charge status of the battery. Generation of the marking pulses is dependent upon stimulation pulses generated by the pacemaker. The system apparently allows for transmission of only one bit of information during any given cardiac cycle, and, as such, is extremely limited in terms of data rate.
The Buffet patent discloses a digital telemetry system intended to provide the capability of checking the actual settings of a large number of programmable parameters having a large range of possible settings, i.e., 16 possible values for any parameter in the disclosed embodiment. A parameter value is assigned a time .DELTA.T which is a fraction of the pacing period T (the cardiac cycle), and then two successive pacing intervals with a combined duration of 2 T are altered by shortening one and lengthening the other by the amount .DELTA.T. The parameter value is identified externally by comparing the relative lengths of the two altered intervals. Telemetry is performed only with the pacemaker operating in asynchronous mode at a predetermined pacing rate. The actual data rate is quite low in that two full cardiac cycles, e.g., two seconds at 60 pulses per minute, are required for transmission of a single digital value. Such a data rate is unacceptably low for many applications of telemetry of data from an implantable device.
Thompson et al. discloses an asynchronous telemetry system for pacemakers in which the interval between transmitted pulses corresponds to the amplitude of an analog signal. As the analog voltage varies, the pulse interval, controlled by a variable-frequency oscillator, varies in a range between 600-1000 microseconds, as illustrated by the waveform of FIG. 3 of that patent. Oscillator-triggered bursts of RF energy from a tank circuit are coupled to an external receiver. The oscillator is free-running.
Another asynchronous telemetry system operating with a free-running oscillator is disclosed in the Brownlee et al. patent cited above, which mentions pulse-interval modulation primarily for telemetry of battery voltage, and also mentions further modulation of the carrier frequency at higher modulation rates for telemetry of refractory delay time and pacing pulse amplitude and/or width. Similarly, in the patent to Johnson, the interval between pacer pulses is indicated as a measure of the status of the pacer battery. Ryan et al. discloses an asynchronous telemetry system in which the intracardiac electrogram is applied as the control signal to a voltage-controlled pulse generator.
Several of the above-referenced patents, such as Silvian, Duggan, Baker, and Thompson et al., describe systems capable of selective transmission of either analog or digital information from a pacemaker. Thompson et al. also mentions a time division multiplexing scheme for sequential transfer of more than one channel of analog data, such as atrial and ventricular electrograms from a dual-chamber pacemaker. The system of Thompson et al. shifts a nominal oscillator frequency to accommodate a second analog channel, and thereby produces an additional range of pulse intervals which need to be measured in the external receiver to demodulate the information on the second channel. The system appears to complete the transmission of digital data before initiating transmission of analog data.
In spite of the existence of numerous techniques for telemetry of analog and digital data from implantable medical devices, there remains a need for a better overall solution to the various problems faced in the design of such telemetry systems, such as the need for extremely low power consumption, operation from a low-voltage source, minimum circuit parts count, circuit reproducibility, compatibility with active devices available for such applications, and acceptable digital data rates and/or practical analog channel bandwidths.